Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates to methods for producing gears through consolidation of metallurgical powders. The present invention also relates to gears produced by such methods. Gears that may be produced by the method of the present invention include, for example, straight gears, helical gears, pinion gears, ring gears, and spur gears.
2. Description of the Invention Background
The production of articles, including gears, from metallurgical powder is well known. Such articles are commonly referred to as xe2x80x9cpowder metalxe2x80x9d articles. Metallurgical powder includes one or more alloyed and/or unalloyed metal powders. Metallic and/or non-metallic additives also may be included. In the usual case, combinations of metal powders and optional additives are mixed to provide a generally homogenous powder blend. A portion of the powder blend is disposed in a mold cavity. The mold cavity has the shape of the desired preform and, for example, may be a simple cylindrical shape or a complex shape approximating the desired final part shape. Pressure is applied to consolidate the metallurgical powder and create a green preform. The preform may then be sintered to fuse the powder particles and increase the density of the compact. In some cases, the sintered part may be in a final form. Typically, however, the sintered part is subjected to one or more closely controlled steps of additional processing to increase part density to more closely approach theoretical full density, impart desired mechanical properties, and/or adjust part dimensions. Possible additional processing techniques include, for example, repressing, resintering, sizing, coining, shot peening, grinding, rolling, and heat treating.
In many cases, parts that may be formed by powder metal techniques also may be formed by machining wrought material (i.e., material prepared by cooling molten material). Various machining techniques may be used to form gears form wrought material. As described in Volume 16, xe2x80x9cMachiningxe2x80x9d, of the ASM Handbook (ASM International 1989), possible machining techniques for producing gear tooth configurations on wrought gear blanks include milling, broaching, shear cutting, hobbing, gear shaping, and rack cutting. Milling, hobbing, shaving, honing, grinding, and rack cutting are techniques commonly used for producing teeth on helical gears.
Hobbing is a generating process in which both the cutting tool and the workpiece revolve in a constant relation as the hob is fed across the face width of the gear blank. The hob is a fluted worm with form relieved teeth that cut into the gear blank in succession, each in a slightly different position. Instead of being formed in one profile cut, as in milling, the gear teeth are generated progressively by a series of cuts. Hobbing is extensively used for forming the teeth of helical gears. The rotation of the workpiece is retarded or advanced, through the action of the cutting machine differential, in relation to the rotation of the hob, and the feed is also held in definite relation to the workpiece and the hob. The extent by which the workpiece is retarded or advanced depends on the desired helix angle.
A conventional technique for producing external helical gears is schematically shown in FIG. 1. The method includes cutting to length wrought steel bar stock, forging toroidal gear blanks from the individual sections, machining the internal diameter, hobbing helically disposed gear teeth on the outer surface of the gear blanks, and then either shaving or rolling the teeth to adjust the dimensions and increase the uniformity of the gear teeth. The gears may then be heat treated and tempered to improve hardness and, possibly, other mechanical properties. The internal diameter is then ground. Grinding, honing, or lapping techniques may then be used to further improve the quality of the gear teeth. Honing of steel gear teeth is often used to remove nicks and burrs, to improve surface finish, and to make minor corrections in tooth shape. Lapping may be used for sets of hardened steel gears that must run quietly.
A level of quality may be assigned to a gear based on the DIN classification system. A classification system for assigning a whole number grade to the level of dimensional accuracy of cylindrical gears is provided in DIN standard 3962. DIN 3962 assigns lower grade numbers to gears having smaller deviation in dimensional characteristics, such as face width and face diameter, that may affect the gear""s alignment with mating parts. The quality grade xe2x80x9c1xe2x80x9d is assigned under DIN standard 3962 to cylindrical gears having the smallest deviation in those characteristics. Thus, cylindrical gears of a particular grade based on the DIN 3962 standard may be produced by setting allowable manufacturing tolerances in line with DIN standards. Those of ordinary skill may readily determine the grade number for a particular cylindrical gear under the DIN 3962 standard by measuring deviations in the relevant gear characteristics or by knowing the tolerances for those characteristics applied during gear manufacture.
One known process for manufacturing external helical gears for automotive applications from wrought steel bar stock includes the above-described sequence of steps. A steel commonly used in that process includes, in weight percentages, 0.18-0.22 iron, 0.60-0.95 manganese, 0.15 max. silicon, 0.35-0.75 nickel, 0.35-0.65 chromium, 0.015-0.045 aluminum, 0.15-0.25 molybdenum, and incidental impurities. As indicated in FIG. 1, gears resulting after the teeth of the hobbed gear blank are shaved qualify as grade 7 based on a comparison of the DIN 3962 standard and the dimensional deviations present in the shaved gear. If the hobbed gear teeth are rolled rather than shaved, then under the DIN 3962 standard the rolled gear typically qualifies as grade 8. The higher grade number indicates that there is somewhat more dimensional deviation in external helical gears produced by rolling. Heat treating the hobbed or shaved gears introduces stresses that affect the dimensional variability of the gears and increases the DIN 3962 grade, usually to grade 9. In applications requiring higher dimensional accuracy and, conversely, lower alignment deviation, the heat-treated gear teeth may be honed to increase the DIN 3962 quality of the gears to about grade 7. If even greater dimensional accuracy is required for a particular application, the time-consuming step of grinding the teeth may increase the DIN 3962 quality to grade 5-6.
In general, the machining steps required to form the teeth and internal diameter of gears produced from wrought material are costly and time consuming. Finishing treatments applied to adjust the dimensions and reduce the dimensional variability of the gear teeth, such as honing and grinding, are particular costly. Applying such finishing treatments to non-linear gears, such as helical gears, is particularly costly, may require the use machinery that is specialized to accommodate the geometry of the gears, and adds significant processing time.
Accordingly, the need exists for a method of economically manufacturing high quality helical gears and other types gears.
In order to address the above-described needs, the present invention provides a novel method for producing gears from an iron-base metallurgical powder that is an alternative to producing the gears from wrought bar stock. The method includes molding at least a portion of the iron-base metallurgical powder to provide a gear preform, and subsequently sintering the gear preform to form a sintered preform. The gear preform is subsequently hot formed, and is carburized in a later step to introduce carbon into at least a surface region of the preform. The gear preform is subsequently resintered and is then cooled at a cooling rate that provides a bainitic microstructure in at least a surface region of the preform. If desired, the gear preform may then be shaved to adjust the dimensions and increase uniformity of the gear teeth.
The present invention also addresses the above-described need by providing a method for producing a gear from a metallurgical powder wherein a gear preform is provided by molding at least a portion of an iron-base metallurgical powder in a gear-shaped mold cavity. The green preform is subsequently sintered. In a later step, the gear preform is hot formed. The hot formed preform is subsequently resintered and is then cooled so as to provide a bainitic microstructure in at least a surface region of the preform. The preform must include sufficient carbon and/or suitable alloying additions so that the desired bainitic structure will result on cooling at an appropriate rate after being held at the resintering temperature. Carbon may be introduced into the preform through a carburizing or equivalent operation, or suitable carbon levels may be provided in the original metallurgical powder so that a carburizing operation or an equivalent operation is unnecessary. The gear teeth of the preform are then shaved to provide a dimensionally improved gear.
Although the foregoing passages generally describe methods considered to be within the present invention, it will be understood that the methods may include additional steps. Such additional steps may be, for example, intermediate and/or downstream of the described steps.
The present invention is also directed to gears such as, for example, external straight gears and external helical gears, produced by the methods of the present invention. More fundamentally, the present invention is directed to gears produced by a powder metal manufacturing method and wherein at least a surface region of the gear preform has a bainitic microstructure. The invention also is directed to articles of manufacture including one or more of such gears. Such articles of manufacture may include, for example, engines, automatic transmissions, four wheel drive transfer cases, and riding lawnmower transmissions.
The methods of the present invention may be used to form gears of any type, including, but not limited to, straight gears, helical gear, pinion gears, ring gears, and spur gears. The present invention""s methods are particularly suited to producing external gears such as external straight gears and external helical gears. Gears produced by methods according to the present invention may be of high quality, qualifying as DIN grade 5 or better under the DIN 3962 standard, immediately after a step of shaving the gear teeth and without grinding, honing, or other surface finishing steps. The powder metal methods of the present invention may require fewer steps and less manufacturing cost than certain conventional methods of producing gears for like applications.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon considering the following detailed description of embodiments of the invention. The reader also may comprehend additional advantages and details of the present invention upon carrying out or using the invention.